1. Field of the Invention
The present invention relates to mass spectrometry of samples with complex molecules, and more particularly to mass spectrometry that simultaneously detects direct (“linear”) and reflected time-of-flight mass spectra.
2. Description of the Related Art
The past approaches described in this section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in this section are not to be considered prior art to the claims in this application merely due to the presence of these approaches in this background section.
Populated locations are susceptible to natural and artificial infestations of biological agents that are harmful to human health. Early detection of such infestations allows rapid evacuation of such locations and provides one line of defense. Rapid treatment of exposed individuals provides a second line of defense. Correct treatment often depends on rapid, correct identification of the harmful agent.
One method commonly used to detect and identify biological agents is mass spectrometry, in which a distinctive distribution of molecular weights is associated with each of several biological agents of interest in protecting public health.
In mass spectrometry, a sample of material is ionized, which changes molecules in the sample to ions (molecules with a net electrical charge). For example, a laser can be used to remove electrons from the molecules, leaving positive ions. The ions are accelerated in a source region using an electric field. For a given electrical voltage used to accelerate the ions, the less massive ions are accelerated to faster speeds than are the more massive ions. Outside the source region, in a region called a “drift region,” each ion travels at a characteristic speed inversely related to its mass. Therefore, the times of flight for the ions to travel from the source region to a detector are related to the masses of the ions. To reduce collisions with air molecules, the source region and drift region are in a vacuum that can be readily produced in a vacuum chamber or that is ambient outside the earth's atmosphere.
Because some of the molecules of interest are rather large, a difference of a few atomic mass units between two molecules, related to a difference in chemical and biological properties, is associated with a relatively small difference in mass and therefore a relatively small difference in speed. To distinguish two molecules that are close in mass and speed, a rather long path for the ions is desirable to increase the difference in time of flight.
Problems arise when the path length is increased. For example, paths of the ions tend to diverge in the drift region; and, thus, more ions miss the detector, decreasing the signal at the detector. To reduce divergence, the ions are focused into an ion beam during the acceleration stage. Focusing the ions into an ion beam reduces the number of ions that miss the detector.
Furthermore, increasing the path length involves increasing the length of the drift region, which increases the size of the mass spectrometer. A larger mass spectrometer is a disadvantage and can cause the mass spectrometer to be too large for some applications. For example, a mass spectrometer that is too large may be unsuitable for a portable unit, or unsuitable for deployment in aircraft, air ducts, and other useful places. The problems are exacerbated if the ion detector is also made larger to compensate for the greater divergence over the longer paths.
In another approach, the path length is essentially doubled without appreciably increasing the size of the mass spectrometer by reflecting the ion beam in a reflecting electric field. The reflecting electric field is tuned to the accelerating field in the source region so that the ions reverse direction after traversing the length of the reflecting region and before striking the end wall of the spectrometer. In conventional reflected time-of-flight mass spectrometers, a directional detector is placed close to the source but facing away from the source. A hole in the detector allows most of the ions in the ion beam from the source to pass through the detector into a reflection portion of the spectrometer. When the ions in the beam are reflected to move back toward the source, many of the ions strike the detector.
A problem with the reflected time-of-flight mass spectrometer is that some very large molecules are too fragile to be decelerated to zero and accelerated into the reverse direction without breaking apart into two or more fragments. For example, molecules with masses of about 10,000 atomic mass units (amu) or higher tend to fragment in a reflected time-of-flight mass spectrometer (an atomic mass unit is about the mass of a proton). One or more of the fragments may be uncharged. An uncharged fragment most likely strikes a wall of the mass spectrometer without ever impinging on the detector and might not be detected even if it does strike the detector. The mass of the fragment becomes lost to the detector. Lighter fragments that retain a charge will be reversed too quickly and strike the detector after a time of flight associated with lower mass molecules.
Another problem with a reflected time-of-flight mass spectrometer is that an incentive to make the detector hole large enough to pass most of the ion beam conflicts with a motivation to make the detector hole as small as possible to detect most of the reflected ions. As a result, the hole is often so small that a significant number of ions are lost that strike the back of the detector and never enter the reflection region. This decreases the signal at the detector.
To be useful as a line of defense in populated areas, the mass spectrometer should detect harmful biological agents with few, small samples that can be filtered from the air or water serving the populated areas. False alarms, caused by detections based on noisy data, should be avoided. A system that repeatedly fires a warning when no danger is actually present is more likely to be ignored when a real threat is detected. To reduce false alarms multiple measurements should be made that verify the existence of the mass distributions upon which detection is based. Well known statistical tests can be performed to generate confidence limits on the detections. Statistical confidence is achieved only after several samples are independently measured.
A problem with conventional mass spectrometer is that so much time is consumed in making several independent measurements that many more people are exposed to the agent before an alarm can be fired. This diminishes the conventional mass spectrometer's effectiveness on one line of defense. For example, to prepare one sample or set of samples for introduction to the mass spectrometer, to introduce the set of samples, to evacuate the air from the vacuum chamber and to remove the spent set of samples can take several minutes. To obtain measurements from even two sets of samples doubles that time and increases the exposure of the population to the biological agent.
Furthermore, it can be difficult to obtain enough independent measurements when sample amount is scarce. It may be difficult to obtain a sample of the biological agent, so that any sample obtained is precious. There may not be sufficient sample to make two independent measurements.
In one approach that may be pursued, a second detector may be placed in the vacuum chamber of a reflected time-of-flight mass spectrometer on a side farthest from the source. Then, before or after a reflected time-of-flight measurement, the reflecting electric field can be turned off, and a direct time-of-flight measurement can be made. However, this approach still loses signal from ions that miss the hole through the plate for the reflected ion detections, and consumes more sample and more time than are used during the reflected time-of-flight measurement alone.
Based on the foregoing there is a clear need for a portable mass spectrometer that can make reliable detections of biological agents, having reduced false-alarm rate, with few samples of small size in a short time. In particular, there is a need for a mass spectrometer that can simultaneously measure direct and reflected time-of-flight mass distributions.